When to start TTC #3??

My younger daughter is going to be 9 months old this Sunday (Big Sister is 9 years old) and I'm starting to think it might be time to start TTC #3. We tried for over a year before I got pregnant with Baby Sister. I don't want to wait forever to TTC, because I know it may take a while, but I don't necessarily want two under two (if, by some miracle, it happens right away). Any advice?? TIA